This invention relates to a simple means for reducing the abrasivity of abrasive pigments such as calcined clay and titanium oxide pigments used by the paper industry.
Calcined kaolin and titania pigments have been used for several decades in a number of industrial applications such as paper coating, paper filling, paints, plastics, etc. In those applications they impart to the finished products a number of desirable properties: brightness, opacity and hiding power. In the case of calcined kaolin pigments, paper coating and filling applications require almost exclusively ultrafine fully calcined kaolin pigments such as ANSILEX 93.RTM. pigment manufactured by Engelhard Corporation. See, for example, U.S. Pat. No. 3,586,523, Fanselow, et al., the teachings of which are incorporated herein by cross-reference.
These fully calcined pigments have an undesirable property, namely, they are relatively abrasive when compared to noncalcined (hydrous kaolin pigments) or, in some cases, partially calcined (metakaolin) pigments. For example, conventional so-called "low abrasion" calcined kaolin pigments typically have an Einlehner abrasion value of about 20 mg. An ultrafine calcined kaolin pigment which has recently been introduced to the paper industry has a lower abrasion but appears to have lower scatter properties than that of the somewhat coarser but more abrasive products such as ANSILEX 93.RTM. pigment. In practical terms, increased abrasivity translates into increased wear of bronze web forming screens (wires) on paper making machines, dulling of paper slitter knives, wear of printing plates when they come into contact with coated paper containing fine calcined pigments in the coating formulation, and, in general, wear of any surfaces coming into contact with these pigments. Titanium dioxide pigments are generally significantly more abrasive than fully calcined kaolin pigments.
Paper makers are becoming increasingly demanding in their need for lower abrasion. To overcome the problem, suitable kaolin can be calcined at temperatures less than those required to produce fully calcined pigments. In this instance, calcination temperature is controlled to produce the form of pigment known as metakaolin. It is known, however, that the brightness of a metakaolin pigment is always poorer, generally by about 2-3%, than that of fully calcined pigments derived from the same clay calciner feed. However, even metakaolin pigments may be more abrasive than paper makers may desire. Examples of patents disclosing calcination of kaolins to provide pigments include: U.S. Pat. No. 3,586,523, Fanselow, et al.; U.S. Pat. No. 3,014,836, Proctor; U.S. Pat. No. 3,058,671, Billue; U.S. Pat. No. 3,343,943, Billue; U.S. Pat. No. 3,171,718, Gunn, et al.; U.S. Pat. No. 4,381,948, McConnell, et al. and U.S. Pat. No. 5,112,782, Brown, et al. Many of these patents make reference to the desirability of reduced abrasivity but in all cases achieve such result by kaolin crude selection and/or processing conditions utilized in steps carried out prior to the final calcination of the original hydrous kaolin feed.
Originally kaolin pigments were supplied in dry form to the end users. The demand for pigments in aqueous slurry form developed with the availability of appropriate shipping and handling facilities. Since the cost of shipping water is an undesirable expense, it is desirable to provide pigment slurries as concentrated as is feasible, consistent with the necessity of formulating slurries that are sufficiently fluid to be pumped yet are resistant to settling. This posed no significant problem when formulating slurries of hydrous (uncalcined kaolins). Thus, using conventional anionic clay dispersants such as condensed phosphate salts and conventional clay handling equipment, 70% solids slurries of fine particle size coating grade hydrous kaolin were readily achieved.
With the advent of ultrafine calcined kaolin pigments such as ANSILEX.RTM. pigment, there was a greater challenge to prepare higher solids slurries of the pigments. However, the achievable high solids were limited by the requirement to assure satisfactory rheology and resistance to settling. In fact, 50% solids slurries of such pigments were difficult to achieve. When solids content increases, both high shear viscosity and low shear viscosity tend to increase. A higher low shear viscosity (such as a high Brookfield) and a lower high shear viscosity (such as a lower Hercules) are desirable. The high shear viscosity could be an indicator of pumpability and the low shear viscosity could be an indicator of settling. The dispersants commonly used usually give a low Brookfield viscosity, which could be an indication of fast settling tendency. When the Hercules viscosity becomes too high (meaning small rpm number) the slurry is not pumpable.
U.S. Pat. No. 5,028,268, Ince, et al., summarizes prior art efforts to improve rheology of aqueous slurries of calcined kaolin. This patent teaches spray drying a slurry of calcined kaolin powder and forming an aqueous slurry from the spray dried powder. Slurries containing 58% and 60% solids are described. Among the references cited in this patent is U.S. Pat. No. 4,017,324, Eggers. In accordance with this patent hydrous kaolin was added along with calcined kaolin in a slurry which also included a suspending agent such as CMC along with a dispersant. This was a development subsequent to the proposal in U.S. Pat. No. 3,130,063 to Millman, et al. to add an organic polymer to a predispersed suspension of the coarse hydrous clay to prevent settling. In accordance with the teachings of Eggers, slurries of mixtures of calcined and hydrous clay having 50% to 75% solids were reported. The dispersant used in illustrative examples was a mixture of relatively large amounts of TAMOL 731 (sodium salt of polymeric carboxy acid) and TRITON X-100 (octylphenoxy polyethoxyethanol). Although the presence of appreciable hydrous clay would be expected to result in a pigment mixture having lower abrasion than that of the calcined kaolin constituent, this approach necessitates an undesirable dilution of calcined clay with a material that is not to calcined kaolin terms of paper sheet properties. Another approach to handling calcined clay pigments was to use sufficient alkalizer agent to provide a pH in excess of 8 to render the suspension free from settling (See U.S. Pat. No. 3,846,147, Tapper). Properties of suspensions having up to 57% solids are described in that patent. In illustrative examples large amounts (30#/ton) of NOPCOSANT L and K were used to produce 58% solids suspensions. (NOPCOSANT is the trademark for anionic polymeric dispersants, namely, sodium salt of condensed naphthalene sulfonic acid.)
U.S. Pat. No. 5,034,062, Lein, et al., discloses the use of acid-containing anionic emulsion copolymers as calcined clay slurry stabilizers. The patent mentions the steep rise in high shear viscosity at solids levels above 50%. This patent broadly discloses that anionic and nonionic surfactants can be included in the slurries when it is desirable to produce dispersed slurries.
Slurries of fine particle size calcined kaolin pigments containing about 50% solids have been commercially available for several years and require the use of organic polymers to prevent sedimentation. The "normal" procedure of making such slurries of calcined kaolin slurries is first to make an aqueous solution of a thickening agent, such as CMC (carboxymethyl cellulose), in water containing a microbiocide, then add a dispersant, such as C211 (sodium polyacrylate) and mix the slurries in a high shear mixer.
Other patents relating to the production of slurries of calcined kaolin clay or calcined pigments derived from kaolin clay are U.S. Pat. No. 3,582,378, Miller and U.S. Pat. No. 5,006,574, Sennett, et al., both assigned to the assignee of the subject patent application.
Thus, the kaolin industry has spent many years in developing calcined kaolin pigments having lower abrasion and has expended a significant effort in producing concentrated slurries of calcined kaolin pigments that are stable. Various materials identified as surfactants or dispersants have been proposed in the development of stable slurries. To the best of the inventor's knowledge, however, none of the efforts directed to reducing abrasion have utilized surfactant addition to previously calcined kaolin and none of the research and development efforts addressing slurry stability focused on abrasion. The problem of pigment abrasion is unrelated to the rheology problem. Nonetheless, an improvement in pigment abrasion that is adverse to the provision of a stable, nonsettling slurry would not be commercially viable.
The present invention arose from the inventor's theory that addition of certain surfactants in very small amounts would reduce the abrasion of abrasive pigments such as calcined kaolin by reducing the friction between the pigment and wires and cutting blades used in paper making. This novel approach to a solution to the abrasion problem is believed to be nonobvious from the prior art. By the present invention, lower abrasion is achieved without the need to add a low abrasion uncalcined kaolin and thereby reduce the optical performance which would occur in practice of the pigment dilution process described in U.S. Pat. No. 4,017,324 (supra).